Invalid object when call createOrGetStripeCustomer

When calling the createOrGetStripeCustomer() function in Cashier’s Stripe integration, I get an “Invalid object” error.

Here’s my Billable model:

/**
 * Get the customer address that should be synced to Stripe.
 */
public function stripeAddress(): string|null
{
    return $this->address_line1 . ' ' . $this->address_line2 . ' ' . $this->postal_code . ' ' . $this->country;
}